Suspension dampers, such as shock absorbers, have been used for many years to control the ride quality of automotive vehicles. In many vehicles, it is desirable to control suspension stiffness. Recently, magneto-rheological fluids have become available for use in vehicle suspension dampers. Magneto-rheological fluids permit the viscosity of the damping fluid to be changed in response to an applied magnetic field. Ride stiffness may thereby be controlled by controlling current in an electric coil within the damper. In magneto-rheological suspension systems, a magnetic field is generated and is applied to the magneto-rheological damping fluid, thereby permitting the viscosity of the fluid to be modified depending upon ride conditions. Accordingly, the stiffness of the suspension system may be easily controlled.
Magneto-rheological suspension systems require a piston rod and a suspension piston in which a coil is mounted. The coil circumscribes the piston rod, and the fluid is communicated through passages circumscribing the piston rod between the coil and the piston ring or skirt. The piston includes a piston skirt or ring, which is held in place by a piston plate which is mounted on the piston rod and supports the piston ring. Accordingly, in order that the magnetic field be applied to the magneto-rheological fluid communicated through the passages, the portion of the piston rod extending through the coil and the piston skirt or ring must be made of a magnetically soft material exhibiting high magnetic permeability and high saturation magnetization. The piston plate, which extends between the piston ring and the piston rod, is desirably a magnetic insulator exhibiting low magnetic permeability and serves only to hold the other components in place. Accordingly, the rod and piston ring define a magnetic circuit in which the electrodes are the piston rod and piston ring. This magnetic circuit is energized by the coil and the magnetic field is generated by electrical current flowing through the coil.
Since the piston ring must be made of a material having high magnetic permeability and the piston plate is desirably a magnetic insulator having extremely low magnetic permeability, it is desirable to manufacture the piston ring and rod out of low carbon soft magnetic steel, while the plate is desirably made out of non-magnetic stainless steel. Because the plate and piston ring are made out of materials having different properties, joining the plate to the ring has been difficult. Conventional welding and brazing techniques have proven to be unreliable. Welding dissimilar materials of the type used in the piston ring and plate disclosed herein is inherently difficult and often results in inconsistent and cracked or failed welds.